Conventional AC-operated, negative glow fluorescent lamps include a light-transmitting envelope that is filled with an inert gas, such as a mixture of 99.5% neon and 0.5% argon at a pressure of about 2 torr, and approximately 12 milligrams of mercury. The inner surface of the lamp envelope is coated with a fluorescent phosphor. Two tungsten filament electrodes coated with an electron emissive coating are mounted within the lamp envelope. A pair of electrical leads connected to each electrode pass through the lamp envelope and are connected to a power supply. The power supply provides low voltage heater current for each of the electrodes and provides an AC operating voltage between the two electrodes.
During lamp operation, a negative glow discharge is set up between the electrodes. During the two halves of the applied AC voltage cycle, the electrodes alternately act as cathodes and anodes. Electrons emitted from the electrode which is acting as a cathode are accelerated by the cathode fall potential and excite mercury atoms in the lamp, thereby causing emission of ultraviolet radiation. The ultraviolet radiation excites the phosphor coating on the lamp envelope which, in turn, emits visible light of a desired color.
Several disadvantages are associated with conventional AC negative glow fluorescent lamps as described above. Electron impingement upon the electrode which is acting as an anode causes locally high temperatures and volatilization of the emissive coating. Lamp lumen output degrades due to deposition of emissive coating substances on the phosphor layer, and lamp life is shortened by loss of the required emissive coating from the electrodes. In addition, a relatively complex and costly power supply is required, and a four-connection interface between the lamp and the power supply is required.
A prior art glow lamp is disclosed in U.S. Pat. No. 3,814,971, issued June 4, 1974 to Bhattacharya. Beam mode fluorescent lamps are disclosed in U.S. Pat. No. 4,408,141, issued Oct. 4, 1983 to Byszewski et al; U.S. Pat. No. 4,413,204, issued Nov. 1, 1983 to Byszewski et al; U.S. Pat. No. 4,450,380, issued May 22, 1984 to Proud et al; U.S. Pat. No. 4,518,897, issued May 21, 1985 to Proud et al; U.S. Pat. No. 4,516,057, issued May 7, 1985 to Proud et al; and U.S. Pat. No. 4,751,435, issued June 14, 1988 to Roche et al.
DC-operated, negative glow fluorescent lamps are also known in the art. Such lamps include an electron-emissive cathode and an anode. A heater current is supplied through the cathode, and a DC voltage is applied between the anode and the cathode. While the cathode emissive coating is not bombarded by electrons in the DC-operated lamp, the power supply for such a lamp is relatively expensive and inefficient. DC-operated, negative glow fluorescent lamps are disclosed in U.S. Pat. No. 4,904,900 issued Feb. 27, 1990 and assigned to the assignee of the present application.
It is a general object of the present invention to provide improved negative glow fluorescent lamps.
It is another object of the present invention to provide AC-operated, negative glow fluorescent lamps.
It is a further object of the present invention to provide AC-operated, negative glow fluorescent lamps in which cathode emissive coating bombardment by electrons is substantially reduced or eliminated.
It is yet another object of the present invention to provide negative glow fluorescent lamps having a long operating life.